WO2012139598A1 - Generation of azimuthally or radially polarized radiation in optical waveguides - Google Patents

Generation of azimuthally or radially polarized radiation in optical waveguides Download PDF

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Publication number
WO2012139598A1
WO2012139598A1 PCT/EP2011/001881 EP2011001881W WO2012139598A1 WO 2012139598 A1 WO2012139598 A1 WO 2012139598A1 EP 2011001881 W EP2011001881 W EP 2011001881W WO 2012139598 A1 WO2012139598 A1 WO 2012139598A1
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WO
WIPO (PCT)
Prior art keywords
azimuthally
grating
optical waveguide
radially polarized
modes
Prior art date
Application number
PCT/EP2011/001881
Other languages
German (de)
French (fr)
Inventor
Andreas TÜNNERMANN
Christoph Jocher
César JAUREGUI MISAS
Jens Limpert
Original Assignee
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Friedrich-Schiller-Universität Jena
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Application filed by Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V., Friedrich-Schiller-Universität Jena filed Critical Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority to PCT/EP2011/001881 priority Critical patent/WO2012139598A1/en
Publication of WO2012139598A1 publication Critical patent/WO2012139598A1/en

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Classifications

    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/10Light guides of the optical waveguide type
    • G02B6/105Light guides of the optical waveguide type having optical polarisation effects
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06Construction or shape of active medium
    • H01S3/063Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067Fibre lasers
    • H01S3/06708Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06712Polarising fibre; Polariser
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06Construction or shape of active medium
    • H01S3/063Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067Fibre lasers
    • H01S3/0675Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B27/00Other optical systems; Other optical apparatus
    • G02B27/28Other optical systems; Other optical apparatus for polarising
    • G02B27/286Other optical systems; Other optical apparatus for polarising for controlling or changing the state of polarisation, e.g. transforming one polarisation state into another
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08Construction or shape of optical resonators or components thereof
    • H01S3/08018Mode suppression
    • H01S3/0804Transverse or lateral mode control, e.g. specifically multimode

Abstract

The invention relates to an apparatus for generating azimuthally or radially polarized radiation by means of an optical waveguide (1), wherein the optical waveguide (1) has a structure for conducting azimuthally or radially polarized modes (5, 7). The invention proposes that the azimuthally or radially polarized modes (5, 7) in the optical waveguide (1) have different effective refractive indices and, within the optical waveguide (1), a narrow-band grating (2), is arranged, in particular a fibre Bragg grating, which is designed such that the spectral distance between two azimuthally or radially polarized resonant modes (5, 7) is equal to or greater than the associated spectra bandwidth.

Description

 The invention relates to a device for producing azimuthally or radially polarized radiation by means of an optical waveguide, wherein the optical waveguide has a structure which is suitable for guiding azimuthal or radially polarized modes.

Devices which emit radially or azimuthally polarized radiation are of great interest for a variety of applications in the field of science, medical technology, military technology or civil engineering, for example for material processing, microscopy or so-called optical tweezers. These applications require light sources with azimuthally or radially polarized radiation, in particular simple, stable, high-performance and cost-effective sources with high polarization purity.

Optical waveguides, in particular glass fibers, which have a structure which is suitable for guiding different azimuthally or radially polarized modes already exist in the prior art. So far, however, it has not succeeded satisfactorily to produce these modes stable and with little effort within the waveguide.

Only when the waveguide structure permits higher modes can radially or azimuthally polarized modes propagate within the optical waveguide. In particular, in the case of rotationally symmetrical waveguide structures, the two TE 01 and TM 0 i modes should be emphasized, which include the

CONFIRMATION COPY Possess property of azimuthal and radial polarization. The problem is that modes with approximately the same effective refractive index combine with each other, so that from the azimuthally or radially polarized modes, a predominantly linearly polarized beam is formed. In this case one speaks of a degeneration of the fashions. For example, in certain structures, the TE 0 i and TM 0 i modes combine with HE21 modes to form a beam with linear polarization. Waveguide structures that allow such combinations are referred to as "weakly-leading".

To solve this problem, various approaches exist in the prior art. For example, in lightly conducting multi-mode fibers, a radially polarized mode can be deliberately excited by coupling a fundamental mode into the fiber with an offset (see T. Grosjean, D. Courjon and M. Spajer "An all-fiber device for generating and other polarized light beams ", Optics Communications, vol. 203, pp. 1-5, 2002). Also, by a special fiber design, for example according to US 2009/0202191 A1, a radially or azimuthally polarized mode can be generated. The fiber design allows the conversion of an existing linearly polarized beam. In combination with micro-bending and a polarizing filter, power can be coupled within the fiber from the fundamental mode into a respective radially or azimuthally polarized mode.

In the solutions known in the prior art, there is the problem that can not be resorted to standard components. This makes them expensive and experimentally complex. Mandatory is always the use of specially manufactured optical elements, each leading to either radial or azimuthal polarization. The necessary high adjustment effort shows direct influence on the polarization purity of the radiation. Frequently, therefore, only a slight polarization purity can be observed. Furthermore, weak waveguides additionally have the problem that the azimuthally or radially polarized modes combine with other modes and thus the polarization purity of the azimuthally or radially polarized beam is reduced. It is therefore an object of the invention to provide a simple, stable and inexpensive radiation source which generates azimuthally or radially polarized radiation. It is another object of the present invention to provide a device in which a simple way between azimuthal and radial polarization within the waveguide can be changed.

This object is achieved by the present invention in that the azimuthal or radially polarized modes in the optical waveguide have different effective refractive indices and within the optical waveguide a narrow-band grating, in particular a fiber Bragg grating is arranged, which is formed so that spectral distance between two azimuthally or radially polarized resonant modes is equal to or greater than their spectral bandwidth.

The device according to the invention uses the principle of so-called "strong guidance" within the waveguide, where the azimuthally and radially polarized modes have different effective refractive indices and can thus be spectrally separated by a grating.The grating sets the difference in the effective refractive index into a difference However, in practice, the wavelength difference between the modes is very small, and in addition to the different effective refractive indices, it is necessary for the grating to be sufficiently spectrally narrow band to satisfactorily separate the azimuthally or radially polarized modes For optimum separation, the spectral distance between two resonant modes should be at least as large as their bandwidth or larger. <br/><br/> The wavelength difference Δλ, which is generated by a fiber Bragg grating with a grating period Λ, leaves calculate for two different effective refractive indices n e ffi and n e ff2 from the Bragg condition:

Δλ - 2 (n ef fi n eff2 ) Λ.

Since the wavelength difference Δλ is very small, the grating must be sufficiently spectrally narrow band to allow sufficient polarization purity guarantee. If the grating is too broadband, the reflected modes overlap and the polarization purity decreases.

Furthermore, an embodiment of the invention provides that the grating is an inhomogeneous fiber Bragg grating, which is designed so that it converts a mode of the waveguide, in particular the fundamental mode, in at least one azimuthally or radially polarized mode. Due to an inhomogeneous lattice constant of the fiber Bragg grating, a mode conversion can take place within the waveguide. Here, a mode of a certain order is converted by the fiber Bragg grating in a mode of another order. For example, this makes it possible to convert the fundamental mode into a TEoi, a TM 0 1 and a HE 2 i mode.

According to the invention, the optical waveguide with the grating arranged therein may be arranged outside or inside a laser oscillator. In an arrangement of the waveguide outside the oscillator, the radiation of the light source is coupled via optical elements in the strong leading waveguide structure. The grating reflects the light according to the wavelength either as azimuthally or radially polarized mode. By an arranged between the light source and optical waveguide outcoupler, in particular beam splitter or circulator, the azimuthally or radially polarized radiation can be separated from the remaining radiation of the light source. The device according to the invention thus serves as an externally arranged polarization filter.

In the case of an optical waveguide disposed within the oscillator, it is recommended that the optical waveguide be doped with a laser-active material. The laser-active material is excited by the radiation of the light source. The two reflective elements of the oscillator are, on the one hand, the grating arranged inside the optical waveguide and, on the other hand, an optical element with wavelength-dependent reflection behavior, in particular an optical grating or a wavelength filter. Depending on the angle of the external grid, the oscillator only supports one specific wavelength. If this wavelength is matched to the reflection properties of the arranged within the optical waveguide grating occurs within the optical waveguide and thus also within the oscillator, only an azimuthally or radially polarized mode.

The invention further provides that the reflection properties of the grating can be influenced thermally or mechanically. By deliberately heating or cooling the grid or by applying a mechanical force, the reflection behavior of the grid is influenced so that it is possible to switch between azimuthally and radially polarized modes. This results in a significant advantage over the prior art, in which can not be changed between azimuthal or radial polarization without much effort. Furthermore, it is possible to resort to commercially available components.

For the purposes of the invention, the optical grating integrated into the waveguide can be both a reflection grating and a transmission grating. Thereby, the device according to the invention can optionally - depending on the arrangement of the optical structure - perform a mode separation in transmission or reflection. A long-period grating (LPG) is recommended as the transmission grating, which couples the unwanted modes into the fiber cladding so that only the azimuthally or radially polarized mode is guided in the core.

The invention further relates to a method for producing azimuthally or radially polarized radiation by means of an optical waveguide, wherein the optical waveguide azimuthally or radially polarized modes leads. According to the invention, the azimuthally or radially polarized modes in the optical waveguide have different effective refractive indices, the modes being filtered by means of a grating disposed within the optical waveguide such that their spectra do not overlap or overlap only slightly.

Embodiments of the invention will be explained in more detail below with reference to FIGS. Show it:

Fig. 1: an apparatus according to the invention for

Generation of azimuthally or radially polarized radiation; Fig. 2: four different modes in strongly leading (left) and weakly leading (right) waveguides;

Fig. 3: Spectral overlap on a mode of broadband fiber Bragg gratings reflected modes;

Fig. 4: spectral separation on a narrowband fiber Bragg grating of reflected modes; Fig. 5: Mode conversion on an inhomogeneous

 Fiber Bragg grating;

6 shows a device according to the invention arranged outside an oscillator; 7 shows a device according to the invention arranged within an oscillator.

The device according to the invention shown in FIG. 1 consists of an optical waveguide 1, a fiber Bragg grating 2, a light source 3 and a coupling-in optical system 4.

According to the invention, the structure of the optical waveguide 1 must be designed to cancel the degeneracy of the modes, ie, to be "highly conductive." This gives the azimuthal or radially polarized modes different effective indices of refraction the difference between the effective refractive indices of the modes is converted into a difference of the reflection wavelength 2. Since the wavelength difference between the modes is relatively small, the fiber Bragg grating 2 must have a sufficiently narrow band, in order to achieve a high polarization purity of the modes, is the fiber Bragg grating 2 closed broadband, the reflected modes overlap and the polarization purity decreases.

FIG. 2 shows a radially polarized TM 0 i mode 5, two HE 2 i modes 6 and an azimuthally polarized TE 0 i mode 7 in the strongly leading (left) and degenerate (right) states within a rotationally symmetrical waveguide 1.

FIG. 3 shows the case in which the fiber Bragg grating 2 is designed in such a broadband manner in relation to the wavelengths of the resonant modes 5, 6, 7 that the reflected modes 5, 6, 7 overlap. In the illustration on the left, the effective refractive index of the modes 5, 6, 7 is shown, in the illustration on the right, the reflection spectrum of the fiber Bragg grating 2, belonging to the modes 5, 6, 7.

FIG. 4 shows the result of the solution according to the invention, in which the modes 5, 6, 7 are reflected on a spectrally narrow-band fiber Bragg grating 2, so that an overlapping of the reflected modes 5, 6, 7 does not take place. As a result, the strongly leading, rotationally symmetric waveguide 1 with the narrow-band fiber Bragg grating 2 acts as a mode filter, which specifically spectrally separates the TM01 mode 5, the HE 2 i modes 6 and the TE 0 i mode 7.

According to FIG. 5, the device according to the invention is used to convert an existing mode 8 of the waveguide into an azimuthally or radially polarized mode 9, 11. For this purpose, an inhomogeneous fiber Bragg grating 2 is used, through which in the waveguide 1, which in each case have the same wavelength spacing between the fundamental mode 8 and the azimuthally or radially polarized modes 5, 7 propagating in the waveguide, so-called mode conversion takes place. In this case, a mode 8 of one order is converted by the inhomogeneous fiber Bragg grating 2 into another mode 9, 10, 11. In the present case, the fundamental mode 8 of the rotationally symmetric waveguide is converted into the TM 0 i mode 9, the HE 2 i mode 10 and the TE 0 i mode 11. The spectrum shows conversion peaks The described mode filter can be arranged outside or inside an oscillator:

An arrangement outside an oscillator is shown for example in FIG. 6. Here, a narrow-band light source 3 is coupled into the strongly guiding waveguide 1 via a collimating lens, a deflecting mirror, a beam splitter 12 and a coupling optics 4. By the fiber Bragg grating 2 either the azimuthally or radially polarized mode 5, 7 is reflected according to the wavelength. The beam splitter 12 separates the azimuthally or radially polarized beam from that of the light source 3, so that the waveguide 1 with the integrated fiber Bragg grating 2 acts as an externally arranged mode filter. If a broadband light source 3 is used, in the beam direction after the beam splitter 12, a spectral separation must still be performed by an edge filter or an etalon in order to obtain a purely radially or azimuthally polarized beam. An arrangement of the mode filter within an oscillator is shown in FIG. 7. The strongly guiding waveguide 1 is here doped with a laser-active material which is excited by the radiation of the light source 3. The radiation is coupled into the waveguide 1 via a coupling-in optical system 4. The oscillator is formed on the one hand by the fiber Bragg grating 2 and on the other hand by an external grating 13. Depending on the angle of the external grating 13, the oscillator only supports a certain wavelength. If this wavelength is tuned to a resonance wavelength of the fiber Bragg grating 3, only the azimuthally or radially polarized mode will oscillate.

By thermal heating or mechanical force influence also the reflection properties of the fiber Bragg grating 3 can be changed. This makes it possible - without changing the structure - to change between azimuthal and radially polarized modes. Another possibility is the change in the wavelength. Here, the external influences are kept constant on the grid and changed over the wavelength of the light source 3 between azimuthal and radially polarized mode. Although in the exemplary embodiments the reflection spectrum of a fiber Bragg grating 2 was used with priority, the device according to the invention can likewise carry out a mode separation in transmission. In this case, in particular, the transmission grating 2 may be a long-period grating (LPG), which only allows the propagation of an azimuthally or radially polarized mode in the core.

- Claims -

Claims

claims
1. A device for generating azimuthally or radially polarized radiation by means of an optical waveguide (1), wherein the optical waveguide (1) has a structure which is adapted to guide azimuthally or radially polarized modes (5, 7)
characterized in that the azimuthally or radially polarized modes (5, 7) in the optical waveguide (1) have different effective refractive indices, and within the optical waveguide (1) a narrow-band grating (2), in particular a Bragg grating, is arranged, which is designed so that the spectral distance between two azimuthally or radially polarized resonant modes (5, 7) is equal to or greater than their spectral bandwidth.
2. Apparatus according to claim 1, characterized in that the grating (2) is an inhomogeneous grating, in particular an inhomogeneous fiber Bragg grating, which is designed so that it is a mode of the waveguide (1), in particular the fundamental mode (8 ), converted into at least one azimuthally or radially polarized mode (9, 11).
3. Device according to claim 1 or 2, characterized in that the optical waveguide (1) with the grating (2) arranged therein is arranged outside of an oscillator.
4. Apparatus according to claim 1 or 2, characterized in that the optical waveguide (1) is arranged with the grating (2) arranged therein within an oscillator.
5. Apparatus according to claim 3 or 4, characterized in that the optical waveguide (1) is doped with a laser-active material.
6. Device according to one of claims 1 to 5, characterized in that the reflection properties of the grating (2) are thermally or mechanically influenced.
7. Device according to one of claims 1 to 6, characterized in that it is switchable between the generation of azimuthally and radially polarized radiation.
8. Device according to one of claims 1 to 7, characterized in that the grid (2) is a reflection grating, in particular a
Fiber Bragg Grating.
9. Device according to one of claims 1 to 7, characterized in that the grid (2) is a transmission grating, in particular a long period grating.
10. Method for producing azimuthally or radially polarized
Radiation by means of an optical waveguide (1), wherein the optical waveguide (1) azimuthally or radially polarized modes (5, 7) leads, characterized in that the azimuthally or radially polarized modes (5, 7) in the optical waveguide (1) different Have effective refractive indices and are filtered by means of a within the optical waveguide (1) arranged grid (2) so that their spectra do not overlap or only slightly.
PCT/EP2011/001881 2011-04-14 2011-04-14 Generation of azimuthally or radially polarized radiation in optical waveguides WO2012139598A1 (en)

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US14/111,279 US9459403B2 (en) 2011-04-14 2011-04-14 Generation of azimuthally or radially polarized radiation in optical waveguides
EP11718932.4A EP2697874A1 (en) 2011-04-14 2011-04-14 Generation of azimuthally or radially polarized radiation in optical waveguides
PCT/EP2011/001881 WO2012139598A1 (en) 2011-04-14 2011-04-14 Generation of azimuthally or radially polarized radiation in optical waveguides

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US9655519B2 (en) 2014-03-21 2017-05-23 Hypermed Imaging, Inc. Systems and methods for performing an imaging test under constrained conditions
JP6340084B2 (en) 2014-03-21 2018-06-06 ハイパーメツド・イメージング・インコーポレイテツド Compact optical sensor

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US9459403B2 (en) 2016-10-04
EP2697874A1 (en) 2014-02-19

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